498 



PROCEEDINGS OF THE AMERICAN ACADEMY 



This was ascertained by making the side of the vessel constitute one 

 metal of a thermal junction. Between this vessel and the platinum 

 strip, which was made incandescent by a current of from 8 to 9 webers, 

 was placed a thermopile. The face of the thermopile was thus ex- 

 posed to the radiation from a given amount of heated surface at a con- 

 stant temperature, while the other was exposed to the radiation of a 

 given surface of platinum. The faces of the thermopile were provided 

 with the customary cones, and a series of diaphragms of thick card-board 

 extended between the radiating surface of the vessel containing the 

 heated water and the platinum strip. The thermopile was connected 

 with a short coil galvanometer, and was moved until the galvanometer 

 needle came to zero. Tliis arrangement was extremely sensitive, — 

 a movement of a centimeter in the position of the faces of the pile 

 being sufficient to drive the spot of light from the galvanometer mir- 

 ror off the scale, corresponding to a movement of nearly fifty centi- 

 meter scale divisions. There is no diiliculty in effecting a balance as 

 quickly as an ordinary photometric measurement is made. While one 

 observer compares a candle or other source of light with the light from 

 an incandescent strip of platinum, another could make the measure- 

 ments with the thermopile, and could obtain the amount of energy 

 radiated by the incandescent strip in terms of the constant source of 

 heat. It is necessary to reverse the faces of the thermopile, or to piace 

 a second constant source of heat on the same side upon which the 

 incandescent strip is placed. The following table indicates the charac- 

 ter of the results. 



The reduction factor of the galvanometer was .44 in the C. G. S. 

 system. When the photometric indications were the same, the 

 thermopile indicated a large change in the amount of heat received. 

 Thus the heat indications within the range in which the experiments 

 were taken were far more sensitive than the photometric indications. 



